Scrollable bottom sheet with a fixed header


I am looking to create something a bit like this as a prototype and having some issues and wondered if you would be able to help please:

It is a slide up bottom sheet.

The grey is the opacity background
The Your 5 things to do header is fixed
The green is the body which scrolls behind the fixed header

When ever I do it the green shows on the opacity layer.

When I want it to just disappear behind the header.

Hope that makes sense?

Any help would be appreciated. Thanks.